1. Technical Field
The present invention relates to a parameter calculating method, a long-term prediction orbit data providing method, an information providing device, and a positioning system.
2. Related Art
Currently, GPS (global positioning system) is widely known as a positioning system using positioning signals, and is included in a positioning device contained in a cellular phone, a car navigation, or other apparatus. The GPS measures a position by performing positioning calculation to obtain a three-dimensional coordinate indicating the position of a GPS device and clock error based on information such as the positions of plural GPS satellites and the pseudo distances between the respective GPS satellites and the GPS device.
According to the positioning method by the GPS, the satellite information such as positions, speeds, and moving directions of the GPS satellites is obtained based on navigation data such as almanac and ephemeris superimposed on GPS satellite signals transmitted from the GPS satellites, and positioning calculation is performed using the satellite information and time information. Almanac is effective for capturing satellites, but is difficult to be used for positioning calculation due to insufficient accuracy of satellite information. On the other hand, ephemeris is effective for capturing satellites and can be used for positioning calculation as well since satellite information is sufficiently accurate. When positioning is started without ephemeris, ephemeris initially needs to be obtained from the GPS satellite signals. As a result, time to first fix (TTFF) increase signals.
US Patent Application/Publication No. 2002/0188403 and US Patent Application/Publication No. 2005/0212700 disclose a technology of a server client system which predicts long-term ephemeris (long-term prediction orbit data) as ephemeris for a long period such as one week using a server and provides the long-term prediction ephemeris to a positioning device as a client.
The long-term prediction ephemeris is defined by data format similar to that of normal ephemeris, for example. According to this method, a satellite orbit is approximated using Kepler's elliptic orbit model as one of satellite orbit approximation models, and long-term prediction ephemeris is defined based on parameters calculated by the model expression of used in this approximation (hereinafter referred to as “satellite orbit parameters”). Satellite prediction almanac (prediction position data) containing prediction positions as future positions of a positioning satellite predicted in time series at predetermined time intervals is provided by predetermined commercial system. Approximating calculation based on Kepler's elliptic orbit model can be performed by using the satellite prediction almanac.
According to a long-term prediction ephemeris producing method in related art, a target period for producing long-term prediction ephemeris such as one week (hereinafter referred to as “production target period”) is divided into fixed “prediction target terms” such as four hours and six hours. In this case, satellite orbit parameters are calculated by approximating or predicting a satellite orbit for each of the prediction target terms to produce long-term prediction ephemeris. That is, long-term prediction ephemeris is produced with lengths of prediction target terms fixed. It is not a preferable method in some cases, however, to divide a long period such as one week into fixed prediction target terms.